The present invention relates to a method for the production of ribbed pipes, in which method a pressurized plastic material is fed through an extruder into a pipe moulding space defined by a mandrel, an end face of an extrusion sleeve and chill moulds moving in the direction of the central axis of the mandrel and defining a mould cavity surrounding the extrusion sleeve and the mandrel, the inner face of the chill moulds being provided with annular grooves.
German Offenlegungsschrift No. 2 362 444 discloses a method for the production of ribbed pipes wherein a plastic material is fed into a relatively small moulding space under a high pressure. The material can be fed into the moulding space by means of a high pressure, because the space is small and closed.
In order to provide the pipe with a desired shape, the feeding speed of the extruder and the speed of travel of the chill moulds have to be proportioned to each other in a determined way. If the feeding speed of the extruder is too high with respect to the speed of travel of the chill moulds, the pressure within the mould cavity rises to such an extent that the chill moulds are drawn apart from each other in the direction of their periphery. Plastic material is thereby squeezed into gaps formed between the chill moulds, which can be seen as axial ridges in the finished product.
The feeding speed of the extruder may also be too low with respect to the speed of travel of the chill moulds. Thereby the moulding space of the corrugating means is not filled up completely, which can be seen as incomplete ribs in the finished pipe.
In order to obtain pipes as flawless as possible, it is known to observe the pipe produced by the apparatus and alter the proportion between the speed of the extruder and that of the chill moulds as soon as deviations in the shape of the pipe are detected, cf. Swedish Published Specification No. 381 001.
A disadvantage of this method is the slow adjustment, because the adjustment cannot be commenced until the apparatus already has been producing a defective product for some time. Furthermore, the extent of the correction is difficult to estimate, wherefore it may take a long time before the apparatus again produces flawless pipe.